CMOS image sensor and method for fabricating the same

ABSTRACT

A CMOS image sensor and a method for fabricating the same in which color balance is enhanced by forming photodiodes to have a depth varied according to the wavelength of incident light to be received through a color filter layer. The predetermined depth varies, from shallow to deep, as the wavelength of the band of incident light increases, such that the predetermined depth is shallowest for the shortest wavelength, e.g., blue light, of the bands of incident light and is deepest for the longest wavelength, e.g., red, of the bands of incident light.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of Korean Patent Application No.10-2004-0109603, filed on Dec. 21, 2004, which is hereby incorporated byreference for all purposes as if fully set forth herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to image sensors, and more particularly,to a complementary metal-oxide-semiconductor (CMOS) image sensor and amethod for fabricating the same in which the photodiode projection depthis controlled to obtain improved color balance.

2. Discussion of the Related Art

Image sensors are semiconductor devices for converting an optical imageinto an electrical signal and include charge-coupled devices andcomplementary metal-oxide-semiconductor (CMOS) image sensors.

A typical charge-coupled device includes an array of photodiodesconverting light signals into electrical signals, a plurality ofvertical charge-coupled devices formed between each vertical photodiodealigned in a matrix-type configuration and vertically transmittingelectrical charges generated from each photodiode, a horizontalcharge-coupled device for horizontally transmitting the electricalcharges transmitted by each of the vertical charge-coupled devices, anda sense amplifier for sensing and outputting the horizontallytransmitted electrical charges. Charge-coupled devices have thedisadvantages of requiring a complicated driving method, high powerconsumption, and a complicated fabrication process with a multi-phasedphoto process. Additionally, in a charge-coupled device integration ofcomplementary circuitry such as a control circuit, a signal processor,and an analog-to-digital converter into a single-chip device isdifficult, thereby their use hinders the development of compact-sized(thin) products, e.g., digital still cameras and digital video cameras,using such image sensors.

CMOS image sensors, on the other hand, adopt CMOS technology that usescontrol circuit and a signal processing circuit as a peripheral circuitand adopts switching technology which detects an image by allowingoutputs to be sequentially detected using MOS transistors provided incorrespondence with the number of pixels arrayed. Additionally, a CMOSimage sensor uses CMOS fabrication technology, i.e., a simplefabrication method using fewer photolithography steps, and results in adevice exhibiting low power consumption.

In the aforementioned CMOS image sensors, typically, the photodiode isthe active device for generating an optical image based on incidentlight signals. In such a CMOS image sensor, wherein each photodiodesenses incident light and the corresponding CMOS logic circuit convertsthe sensed light into an electrical signal according to inputwavelength, the photodiode's photosensitivity increases as more light isable to reach the photodiode. One way of enhancing a CMOS image sensor'sphotosensitivity is to improve its “fill factor,” i.e., the degree ofsurface area covered by the photodiodes with respect to the entiresurface area of the image sensor. Accordingly, the fill factor isimproved by increasing the area responsive to incident light. Theconcentration of incident light onto the photodiode is furtherfacilitated when the quantum efficiency at all wavelengths (white light)is “1,” which represents a balanced transmission to the photodiodesacross the spectrum to include complimentary components of red light,blue light, and green light received at the photodiodes.

To concentrate the incident light on one or more photodiodes, a deviceexhibiting excellent light transmittance, such as a convex microlens forrefracting incident light, may be provided. The convex microlens is usedto redirect any light that may otherwise be incident to the image sensoroutside the immediate area of the photodiodes. In a color image sensor,such a microlens having a predetermined curvature (i.e., a convex lens)may be provided over a color filter layer for passing the light of eachcolor (wavelength). As shown in FIG. 1, a CMOS image sensor according tothe related art, includes three photodiodes provided for generatingelectrical signals according to the intensity and wavelength of incidentlight.

Referring to FIG. 1, a CMOS image sensor according to the related artincludes a p-type epitaxial layer 11 grown on a p-type semiconductorsubstrate 10 defining a device isolation region and an active region. Afield oxide layer 12 is formed in the device isolation region of thesemiconductor substrate 10 to isolate light-signal input regions of blue(B) light, green (G) light, and red (R) light from one another. First,second, and third n-type regions 13, 14, and 15, to serve as therespective photodiodes of a color image sensor, are formed of equaldepths by ion-implantation in the active region of the semiconductorsubstrate 10. Subsequently, a series of gate electrodes 17 are formed onthe active region of the semiconductor substrate 10 by interposing agate insulating film 16 patterned with the gate electrodes. Dielectricspacers 18 are formed at the lateral sides of each gate electrode 17. Adielectric interlayer 19 is formed over the entire surface of thesemiconductor substrate 10 including the gate electrodes 17. A colorfilter layer 20 comprised of blue, green, and red filters (i.e., a colorfilter array) is formed on the dielectric interlayer 19 to correspond tothe first, second, and third n-type regions 13, 14, and 15. Aplanarization layer 21 is formed over the entire surface of thesemiconductor substrate 10 including the color filter layer 20. Aplurality of microlenses 22 is formed on the planarization layer 21 tocorrespond to the respective color filters of the color filter layer 20.

In the operation of the CMOS image sensor, incident light isconcentrated by the microlenses and received by the photodiodes in eachof the n-type regions. During this light signal reception, light of eachcolor penetrates the silicon layer by a predetermined depth according towavelength, with longer wavelengths, e.g., red light, achieving a deeperpenetration.

Since the photodiodes of the CMOS image sensor according to the relatedart, as described above, are respectively formed of the first, second,and third n-type regions 13, 14, and 15, they each have the sameprojection depth regardless of the intended spectral reception, i.e.,red, green, or blue light. However, because the penetration depth ofincident light is different for the various colors (wavelengths), havingphotodiodes with the same projection depth regardless of the intendedspectral reception results in lower absorption coefficients for colorshaving longer wavelengths, e.g., red light. This variation in absorptioncauses unduly low levels at the red end of the spectrum and thus a colorimbalance. Furthermore, with its longer wavelength and its deeperpenetration, the absorption of red light may extend beyond the lowerlimits of the corresponding photodiode (active) region, and causecrosstalk, i.e., a transfer of charges between adjacent active regions.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a CMOS image sensorand a method for fabricating the same that substantially obviates one ormore problems due to limitations and disadvantages of the related art.

One advantage of the present invention is that it can provide a CMOSimage sensor and a method for fabricating the same, which achieves colorbalance across an arrangement of photodiodes for color image detection.

Another advantage of the present invention is that it can provide a CMOSimage sensor, by which the photodiodes have varied projection depthsaccording to the wavelength of light incident to the corresponding colorfilter of a color filter layer.

Another advantage of the present invention is that it can provide a CMOSimage sensor, by which the projection depth of an active region (thephotodiode) is deeper for longer wavelengths of incident light.

Another advantage of the present invention is that it can provide a CMOSimage sensor, in which crosstalk is reduced.

Another advantage of the present invention is that it can provide asuitable method for fabricating any of the above CMOS image sensors.

Additional advantages, examples of and features of the invention will beset forth in part in the description which follows, and in part will beapparent from the description or by practice of the invention.

To achieve these and other advantages in accordance with an embodimentof the present invention, as embodied and broadly described herein,there is provided a CMOS image sensor having a plurality of activedevices for receiving each of a corresponding plurality of bands ofincident light according to wavelength. The sensor comprises asemiconductor substrate; and an active region for each band of incidentlight, respectively formed over the surface of the semiconductorsubstrate to have a predetermined depth relative to each band ofincident light.

It is to be understood that both the foregoing general description andthe following detailed description of the present invention areexemplary and explanatory and are intended to provide furtherexplanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiment(s) of the inventionand together with the description serve to explain the principles of theinvention.

In the drawings:

FIG. 1 is a sectional view illustrating a related art CMOS image sensor;and

FIGS. 2A-2G are sectional views illustrating process steps offabricating a CMOS image sensor according to an embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to some of the embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. Wherever possible, like reference designations will be usedthroughout the drawings to refer to the same or similar parts. Also,layer thickness, relative proportions, and other dimensions as shown maybe exaggerated or distorted to more clearly depict semiconductorcomponents and materials, including layers, films, depositions, andother areas.

FIGS. 2A-2G illustrate process steps of fabricating a CMOS image sensoraccording to an embodiment of the present invention.

As shown in FIG. 2A, a p³¹ -type epitaxial layer 101 is formed on ap⁺⁺-type semiconductor substrate 100. A monocrystal silicon substrate isused as the semiconductor substrate 100.

As shown in FIG. 2B, a field oxide layer 102 is formed by a shallowtrench isolation process or a local oxidation of silicon process in adevice isolation region of the semiconductor substrate 100 to define anactive region. Subsequently, a gate insulating film 103 and a materiallayer for gate electrode formation are formed on the epitaxial layer101. The gate electrode material may be polysilicon or a metal of athickness of approximately 2000-3000 Å. Portions of the gate electrodematerial layer and the gate insulating film 103 are selectively removedby photolithographic and etching processes, thereby forming gateelectrodes 104 within the active region defined by the filed oxide layer102. The gate electrodes 104 shown are those of a transfer transistorper unit pixel. A layer (not shown) of insulating material is depositedover the entire surface of the semiconductor substrate 100 including thegate electrodes 104 and then etched back to leave dielectric spacers 105formed on the lateral sides of each gate electrode.

As shown in FIG. 2C, photoresist is deposited over the entire surface ofthe semiconductor substrate 100 including the gate electrodes 104 and ispatterned by known exposing and developing processes, to form a firstphotoresist pattern 106 having openings corresponding to where each of aplurality of n-type regions (as photodiodes for receiving blue, green,and red light) are to be formed by ion-implantation. Lightly dopedn-type impurity ions are firstly implanted using the first photoresistpattern 106 as a mask, thereby forming a first n-type region 107 in theexposed surfaces of the epitaxial layer 101. The exposed surfaces extendbetween a point over one of the field oxides to a point over the one ofthe gate electrodes, such that the field oxides and the gate electrodesalso serve as part of the mask. In one embodiment of the presentinvention, the photodiodes may be separated by a constant interval. Itshould be understood that, although the first n-type region 107 will beused for the photodiode receiving blue light, the same ion-implantation(same depth) occurs for each of the other colors.

As shown in FIG. 2D, after the first photoresist pattern 106 is removed,a second photoresist is deposited over the entire surface of thesemiconductor substrate 100 and is patterned by exposure and developmentprocesses to form a second photoresist pattern 108, which has openingsfor the green and red regions only, i.e., photodiodes for receivinggreen and red light. Lightly doped n-type impurity ions are againimplanted using the second photoresist pattern 108 as a mask, therebyforming a second n-type region 109 in the exposed surfaces of theepitaxial layer 101, namely, in the green and red regions, each of whichis thus imparted with an increased depth that is deeper by a distance“g” with respect to the first n-type region 107 (the blue region). Theachieved ion concentration and implantation depth of the n-type impurityions in the green and red regions may depend on the structure and designcharacteristics of the particular device being fabricated. In anexemplary embodiment, the above formation of the second n-type region109 (the green region) achieves a depth that is approximately 10-50%greater than that of the first n-type region 107 (the blue region).

As shown in FIG. 2E, after the second photoresist film 108 is removed, athird photoresist is deposited over the entire surface of thesemiconductor substrate 100 and is patterned by exposure and developmentprocesses to form a third photoresist pattern 110, which has openingsfor the red region only. Lightly doped n-type impurity ions are againimplanted using the third photoresist pattern 110 as a mask, therebyforming a third n-type region 111 (the red region) in the exposedsurfaces of the epitaxial layer 101. By this process, the red region isthereby imparted with an increased depth that is deeper by a distance“r” with respect to the depth of region 109 (the green region) asobtained by the second ion-implantation (FIG. 2D). According to anexemplary embodiment, the distance r is approximately 10-50% of thegreen region's ultimate depth.

Accordingly, in an embodiment of the method for fabricating a CMOS imagesensor according to the present invention, the depth of each photodioderegion, which is the area of the active device, is varied in accordancewith the wavelength of light incident to each region, such that eachphotodiode region has a predetermined depth relative to each band ofincident light among a plurality of bands of incident lightcorresponding to the color filter layer. Thus, the predetermined depthvaries, from shallow to deep, as the wavelength of the band of incidentlight increases, such that the predetermined depth is shallowest for theshortest wavelength of the band of incident light and is deepest for thelongest wavelength of the band of incident light. As such, the blueregion (the n-type region 107) is the shortest, and the red region (then-type region 111) is the longest.

In one embodiment of the present invention, three steps ofion-implantation are performed at equal energy levels, using sequentialmask patterns including aperture sets of three holes (blue, green, andred), two holes (green and red), and one hole (red), respectively. Inanother embodiment of the present invention, three steps ofion-implantation are performed at increasing energy levels, and/or usingsequential mask patterns including aperture sets of one hole (blue), onehole (green), and one hole (red), respectively.

As shown in FIG. 2F, after the third photoresist pattern 110 is removed,a dielectric interlayer 112 is formed over the entire surface of thesemiconductor substrate 100. Subsequently, the color filter layer isformed to have an interlaced plurality of color filters. A blue filter113 is formed on the dielectric interlayer 112 to correspond to then-type region 107, a green filter 114 is formed thereon to correspond tothe n-type region 109, and a red filter 115 is formed thereon tocorrespond to the n-type region 111. In this manner, the blue filter 113is disposed over the n-type region 107 having the least implantationdepth of n-type impurity ions, and the red filter 115 is disposed overthe n-type region 111 having the greatest implantation depth of n-typeimpurity ions. Thus, the blue, green, and red filters 113, 114, and 115for filtering light of each band of wavelengths are sequentially formedby depositing salt resists, which then undergo exposure and developmentprocesses.

As shown in FIG. 2G, a planarization layer 116 is formed over the entiresurface of the semiconductor substrate 100 including the color filterlayer, and a material layer for microlens formation is deposited on theplanarization layer 116. A plurality of microlenses 117, correspondingto the color filters 113, 114, and 115 of the color filter layer, areformed by selectively patterning the microlens material layer and thenreflowing the patterned material layer. An oxide film such as resist ortetra-ethyl-ortho-silicate may be used as the microlens material layer.

In the CMOS image sensor of the present invention, the distance betweenthe n-type region 111 and the semiconductor substrate 100 is theshortest, and the distance between the n-type region 107 and thesemiconductor substrate 100 is the longest. A reduction in the distancebetween the p-type semiconductor substrate and the n-type regions 107,109, and 111 reduces the potential for crosstalk to occur betweenadjacent pixels, which is most severe for the longest wavelength, i.e.,red light. If the distance between the p-type semiconductor substrateand the n-type regions 107, 109, and 111 is reduced, recombination ofelectron hole pairs is actively made during inflow of optical charges toreduce the concentration of drift electrons, thereby reducing crosstalk.Therefore, in an embodiment of the present invention, the overall depthof the field oxide layer 102 is made to be at least equal to thegreatest (i.e., red) depth among the n-type regions.

Accordingly, a CMOS image sensor according to the present invention isshown in FIG. 2G in which the p-type epitaxial layer 101 is grown on thep-type semiconductor substrate 100 defined by a device isolation regionand an active region. The field oxide film 102 is formed in the deviceisolation region to isolate input regions of green light, red light, andblue light. The n-type regions 107, 109, and 111, serving asphotodiodes, are formed of varying depths in the active region. The gateelectrodes 104 are formed on the active region by interposing the gateinsulating film 103. The dielectric spacers 105 are formed at thelateral sides of each gate electrode 104. The dielectric interlayer 112is formed over the entire surface of the semiconductor substrate 100including the gate electrodes 104. The color filters 113, 114, and 115of the color filter layer are formed on the dielectric interlayer 112 incorrespond with the formations of the n-type regions 107, 109, and 111.The planarization layer 116 is formed over the entire surface of thesemiconductor substrate 100 including the color filter layers 113, 114,and 115. The microlenses 117 are formed on the planarization layer 116to correspond to the respective color filter layers 113, 114 and 115.

In the CMOS image sensor and the method for fabricating the sameaccording to an embodiment of the present invention as described above,since the photodiode where the red light enters is formed to be deeperthan another photodiode, characteristics of red signals can be improvedto distribute optical charges caused by the red light in the photodiode.Since the distance between the photodiode corresponding to the red lightand the p-type semiconductor substrate is reduced, crosstalk of adjacentpixels can be reduced.

It will be apparent to those skilled in the art that variousmodifications can be made in the present invention without departingfrom the spirit or scope of the invention. Thus, it is intended that thepresent invention cover such modifications and variations provided theycome within the scope of the appended claims and their equivalents.

1. A CMOS image sensor having a plurality of active devices forreceiving each of a corresponding plurality of bands of incident lightaccording to wavelength, the sensor comprising: a semiconductorsubstrate; and an active region for each band of incident light, whereineach active region is formed to have a predetermined depth relative toeach band of incident light.
 2. The CMOS image sensor according to claim1, wherein the predetermined depth varies, from shallow to deep, as thewavelength of the band of incident light increases.
 3. The CMOS imagesensor according to claim 1, wherein the predetermined depth isshallowest for the shortest wavelength of the bands of incident lightand is deepest for the longest wavelength of the bands of incidentlight.
 4. The CMOS image sensor according to claim 1, wherein each saidactive region is a photodiode.
 5. The CMOS image sensor according toclaim 1, wherein each said active region is an n-type region and saidsemiconductor substrate is a p-type semiconductor substrate.
 6. The CMOSimage sensor according to claim 5, wherein said semiconductor substrateincludes a p-type epitaxial layer and wherein the n-type region isformed in the p-type epitaxial layer.
 7. The CMOS image sensor accordingto claim 1, further comprising: a dielectric interlayer covering eachsaid active region and being formed over the surface of saidsemiconductor substrate; a color filter layer, formed on said dielectricinterlayer, having an interlaced plurality of color filterscorresponding to the respective formations of said active region; aplanarization layer formed on said color filter layer; and a microlensformed on said planarization layer above each color filter of said colorfilter layer.
 8. The CMOS image sensor according to claim 7, wherein theplurality of color filters includes a red filter, a green filter, and ablue filter.
 9. The CMOS image sensor according to claim 8, wherein eachsaid active region is formed as a series of first, second, and thirdphotodiodes corresponding to the blue filter, the green filter, and thered filter, respectively.
 10. The CMOS image sensor according to claim9, wherein the third photodiode has a greater depth than the first andsecond photodiodes.
 11. The CMOS image sensor according to claim 9,wherein the first photodiode has a lesser depth than the second andthird photodiodes.
 12. The CMOS image sensor according to claim 9,wherein the second photodiode has a depth that is approximately 10-50%greater than the depth of the first photodiode.
 13. The CMOS imagesensor according to claim 9, wherein the third photodiode has a depththat is approximately 10-50% greater than the depth of the secondphotodiode
 14. The CMOS image sensor according to claim 9, wherein thefirst, second, and third photodiodes are separated by a constantinterval.
 15. A method for fabricating a CMOS image sensor having aplurality of active devices for receiving each of a correspondingplurality of bands of incident light according to wavelength, the methodcomprising: forming over a semiconductor substrate an active region foreach band of incident light, wherein each active region has apredetermined depth relative to each band of incident light.
 16. Themethod according to claim 15, further comprising: forming a dielectricinterlayer covering each active region over the semiconductor substrate;forming a color filter layer on the dielectric interlayer, the colorfilter layer having an interlaced plurality of color filterscorresponding to the respective formations of each active region;forming a planarization layer on the color filter layer; and forming, onthe planarization layer, a microlens above each color filter of thecolor filter layer.
 17. The method according to claim 16, wherein theplurality of color filters includes a red filter, a green filter, and ablue filter.
 18. The method according to claim 17, wherein each activeregion is formed as a series of first, second, and third photodiodescorresponding to the blue filter, the green filter, and the red filter,respectively.
 19. The method according to claim 18, wherein the thirdphotodiode is formed to have a greater depth than the first and secondphotodiodes.
 20. The method according to claim 18, wherein the firstphotodiode is formed to have a lesser depth than the second and thirdphotodiodes.
 21. The method according to claim 18, wherein the thirdphotodiode is formed to have a depth that is approximately 10-50%greater than the depth of the second photodiode.
 22. The methodaccording to claim 18, wherein the second photodiode is formed to have adepth that is approximately 10-50% greater than the depth of the firstphotodiode.
 23. The method according to claim 18, wherein the first,second, and third photodiodes are formed by three ion-implantation stepsperformed at equal ion energies, using sequential mask patterns havingaperture sets for the first, second, and third photodiodes, for thesecond and third photodiodes, and for the third photodiode,respectively.
 24. The method according to claim 18, wherein the first,second, and third photodiodes are formed by three ion-implantation stepsperformed at different ion energies, using sequential mask patternshaving aperture sets for the first photodiode, for the secondphotodiode, and for the third photodiode, respectively.
 25. The methodaccording to claim 18, said active region forming comprising: depositinga first photoresist film over the semiconductor substrate and patterningthe first photoresist film to define respective regions for the first,second and third photodiodes; implanting impurity ions, at a first ionimplantation energy, into each of the respective regions using thepatterned first photoresist film as a mask to form the first photodiode;depositing a second photoresist film over the semiconductor substrateand patterning the second photoresist film to expose only the second andthird photodiode regions; implanting impurity ions, at a second ionimplantation energy, into each of the exposed second and thirdphotodiode regions using the patterned second photoresist film as a maskto form the second photodiode; depositing a third photoresist film overthe semiconductor substrate and patterning the third photoresist film toexpose only the third photodiode region; and implanting impurity ions,at a third ion implantation energy, into the exposed third photodioderegion using the patterned third photoresist film as a mask to form thethird photodiode, wherein the second ion implantation energy is higherthan the first ion implantation energy and wherein the third ionimplantation energy is higher than the second ion implantation energy.